Phase modulation receiver



May l5, 1945. M. G. CROSBY 2,376,125

PHASE MODULATION RECEIVER Filed oct. so, 1941 4 sheets-sheet 1 NTOR ATTORNEY M.. m. @ET W..

I May-1 5, 1945.

M. G. CROSBY 2,376,125

PHASE MODULATION RECEIVER Filed Oct. 30, 1941 4 Sheets-Sheet 2 A@ @0% mg. K SNQSIIQ. 96 my "uw Sw .QN @www GORNNUMN k. .Orwnv @MNQ K l BY May15, 1945. Y M GCROSBY 2,376,125

PHASE MODULATION RECEIVER Filed 001'.. 30, 1941 4 Sheets-Sheet 3 INVENTOR Murr@ G. 01:04'33 ATTORNEY May l5, 1945. M GQ CROSBY PHASE MODULATION RECEIVER' i Filed Oct. 30, 1941 A@Sheets-Sheet 4 gwg 7@ W w w05 ATTORN EY .Patented May' 15, 1945 UNITED A's'ricrlzsy' PATENT l OFFICE.

PHASE MODULATION RECEIVER Murray G. Crosby, Riverhead, N. Y., assignor to Radio" Corporation of America, a corporation of Delaware Y Application october so, 1941se`ria1 No. v117,103

This .application concerns a phase modulation receiver in which a frequency modulation converting circuit is utilized for detection and inverse feedback is applied in av manner which corrects the modulation on the incoming wave from phase lmodulation toA modulation havingthe characteristics of frequency modulation. In order to do this, Athe frequency modulation receiver detector output is applied to a phase modulator associated With a precedingstage in a manner to. phase modulate the wavesupplied to the detector. This phase modulation may involve phase modulating oscillations used for heterody-ning purposes in an incoming stage or may involve phase modulatingr the incoming signals in an amplifier through which. the incoming signal passes.

- v- My novel receiver then is of the type ofr phase modulation. receivers wherein a frequency modulation detector is used to dex-nodulatey the modulated carrier and the frequency distortion encountered, `due to the detection of phase modulation in ra frequency modulation detector, is compensated for to producev an'undistorted output. Whenalphase modulated carrier waveA is received and impressed on afrequency modulation ,detector, the inherent difference between frequency modulation and* phase. modulation lcauses the voutputof the frequency modulation detector to have. a frequency characteristic such that the output amplitude is proportional to the frequency of the modulating potentials. In orderltov correct for this characteristic, the receivers ofthe prior `vthe, passedsignals vare, excluded from the feed hack` loop circuit. f In afmodilication the feed back potentialis. derived ina separate detector, lthereby avoiding the defects caused in known receivers of this type by time delay in the feed back circuits.

In describing my invention in detail, reference will be made to the attached drawingsr wherein Fig. 1 illustrates an embodiment of my receiver,

, in which heterodyning oscillations are modulated 14 oiaims. (c1. 25o-'20)v 'f as tophase by potentials derived by cle-modulating phase modulated waves in a frequency modulated `receiver and the so phase modulated oscillations used to heterodyne the received wave toa frequency-commensurate with the tuning of the frequency modulation detector;

Figure la is a graphillustrating the manner in which the correctionis accomplished in the receiver of Fig. 1. YThis figure, alsov illustrates the manner in which the correction is accomplished in the remaining figures; v Figs. 2, 3, 4, and 5 are `modications of the arrangementy of Fig. 1. y

In Figs. 3, 4, and 5 a separate frequency modulation detector is used for obtaining the control potential andthe control potential is modied and operates through a reactance tube on an oscillator to modulate its yfrequency in a sense to compensate'the output of a frequency modulation detector operating on phase modulated waves. In'this arrangement, since a separate detector is used to derive the feedback potential,l the correction maybe accomplished invan early stage of the receiver'with ban-dpass filters following the samey without including such filters in the degeneration circuit' to cause oscillations therein.

The circuitA of Fig. 1 is a fundamental circuit. The signal applied to the input of network rI is amplified' and converted therein to` intermediate frequency. Network I schematically represents a conventional radio frequency yampl-ier and-first detectonitbeing understood that the latter is fed with local oscillations from alocal highfre quency oscillator whereby' the output energy of the first detector is of intermediate frequency (I. FJ. The intermediate frequencyenergy is selectively amplified in band pass I. F. amplifier 2, and fed to heterodyne converter 3', where it is again convertedl in frequency and supplied to a frequency modulation demodulator consisting of a discriminator and vldetector circuit. Beating oscillator 6 feeds its energy through phase modulator 5 and yfrequency multiplier 4 to the heterodyne and converter 3. The second intermediate frequency output of heterodyne converter 13 is fed to the frequency discriminating circuit comprising a transformer 1 and coupling condenser 8 Awhich converts the frequency modulations on the wave energy fed thereto to amplitude modulation for detection by detectors 9 and I0. The phase modulator 5 may be of any known type, as, for example, my U.. S. Patent No. 2,033,231

granted March 10, 1936. Since such circuits, are

well known in`y the arty andsince this circuit per Il are applied to phase modulator 5.

se forms no part ofmy invention, the same need not be described here.

The frequency modulation detector circuit comprising paths 1 and 8 and opposed detectors 9 and I0, is likewise of a well known type shown by S. W.- Seeley in his U. S. Patent No. 2,121,103 granted June 21, 1938, and per se forms no part of my present invention and a description thereof is believed unnecessary. However, it will be noted that at the output of the detectors 9 and l0, tha-t is, across resistances R. and Rl connected in series between the cathode of` detector 9 and ground, will appear potentials characteristic of the phase modulations on the wave energy being detected, except for the modification thereof referred to above and referred to hereinafter. These potentials may be fed to audio frequency amplifiers, and are utilizing means and also fed through line ll tothe phase modulator 5 to control therein the phase of the oscillations from oscillator 6.

With the phase modulated signal applied to the input of the receiver at network l, the output of the receiver across the resistance R and RI, if line Il were omitted from the circuit of Fig. 1, would have a characteristic as shown by the solid line audio frequency c. ouput characteristic A in Fig. 1a. The polarity of the phase modulation applied to the beating oscillations from 6 by phase modulator 5 is made such as to reduce the degree of phase modulation on the frequency converted output of the heterodyne converter 3 when the audio potentials from line Thus the modulating potentials applied to unmodulate or lessen the degree of modulation on the second intermediate frequency tend to reduce the degree of modulation to a larger extent on a higher modulation frequency by virtue of the fact that the potential applied to line Il has a characteristic as shown by the solid line A of Fig. la. Consequently, the feedback, due to line Il, tends to flatten out the frequency characteristic at the output of the detectors 9 and Ill so that they have a characteristic as shown by the dotted line B of Fig. 1a. Thus the distortion due to the fact that a frequency modulated wave detecting system is used on a phase modulated wave is automatically corrected for.

the frequency multiplier employed in 4.

An alternative to the application of the feedplied to a receiver, which herein will be designated the main receiver, for control purposes to condition the said main receiver, which is of the frequency modulated wave type, to the reception .of phase modulated waves. This arrangement has the advantage that there are no time delays in the feedback circuit which can cause regeneration or sustained oscillations in either receiver. 'Ihe receiver comprises a radio amplifier 48 feeding amplified wave energy to two heterodyning detector systems, the first of which comprises oscillator 4I and detector 42,

back phase modulation to the beatingoscillator I is to pass the phase modulated waves through a phase modulator in an intermediate frequency stage. This in effect eliminates units 3, 4, and 6`,` and includes the phase modulator 5 at the position where 3 is'now shown in Fig. l. Thus the step yof frequency conversion is eliminated and phase modulation is applied directly to the phase modulated carrier of intermediate fretials are obtained in a separate receiver and supand the second of which comprises oscillator 4l and detector 49. The output of detector 42 supplies energy of intermediate frequency to a band-pass intermediate frequency amplifier 43, and from 43 the filtered and intermediate frequency phase modulated wave energy is supplied to a discriminator circuit 44 and thence to detectors 45 and 46 to provide a control potential on line Il as in Fig. 1. This control potential is supplied by way of a potential modifying time constant circuit including condenser 44 and resistance 45 to so modify the potentials that the frequency modulator consisting of the reactance tube modulator 48 and oscillator 4l feeds a phase modulated wave to detector 49. The modulated oscillations are used in detector 49 `to heterodyne the phase modulated wave to a lower frequency in 50 and supply the same to the second frequency discriminator circuit 5I of the main receiver, and from the discriminator 5I to detectors 52 and 53, from whence the output may be supplied to audio frequency amplifiers at utilization means.

It is to be noted that the band-pass filter 50 inserted between heterodyne detector 49 and the frequency discriminator 5| of the main receiver is not includedl in the feedback circuit and therefore cannot introduce time delay to cause regeneration or sustained oscillations. Furthermore, the same can be said of the band-pass filter and amplifier 43 of the receiver for obtaining the control potentials The rst beating oscillator of the circuit of Fig. 1 could not be controlled, since the band-pass filter would be inserted between the detector 3 and the discriminator 1, 8 and would introduce a time delay which limits the degree of feedback which can be used before sustained oscillations are encountered.

In the arrangement of Fig. 3 the beating oscillator 47 is phase modulated iby corrected frequency modulation. By this is meant that the potentials supplied to the reactance tube 48 are so modified byl condenser 44 and resistance 45 that although the, reactance tube 48 vmodulates the frequency of oscillator 4l, the resultant modulated oscillations have the characteristics of phase modulation. Condenser 44 is adjusted so that its audio reactance is high compared to resistance 45 so that the network imparts an attenuation to the lower frequencies thereby causing the output of the network to have an amplitude proportional to the frequency of the input. This is the desired characteristic for changing frequency modulation into phase modulation.

The arrangement of Fig. 3 may be modified as illustrated in Fig. 4 to phase modulate the oscillations for oscillator 41 in 41 and supply them by way of a frequency multiplier 41" to the heterodyning detector 49. The operation of this arrangement would loe as described in connection with Fig. l of the drawings.

The arrangement of Fig. 3 may also be modi- 'stantiall'yudirectly to, said frequency lmodulated.

liedi asl illustrated in Figi:- 5 so L=that?,tlref-phase Imodulated wave energy is supplied to-aflconv'erter comprisingfoscillator-4| anddetector@'42,;r asin intermediate lfrequenc'zy oscillator so i that ;v too much time delay is introduced b'y the band-pass intermediate-frequency amplifier; Tl'liszireduces the-.amount fof feedbackrpossi-blej so @that ylittle f or.

nofrequency correctionvmay nbc:- obtained.k With pressing phase modulated wave-"detector, a..source of ioscillations; aphase modulator coupling said vsource of fioscillations to said'heterodyne-detector; a coupling :between the output of said' frequency .modulated vwave detector and said' phasermodulator, and means -for .imenergyV on said. hetero.-

' dyne detector.

'cdetaifledfdescription thereof. is' Inot 'th'ought'necesc 5:. In a.phasefmodulationreceiver, a frequency modulatednwavef detector, a. frequency lconverting-system; having anfinput excited by said. .wave

energy. and.- by locally .generated oscillations, said convertingsystem`| having: an .output coupled-to 'said frequency` modulated wave detector, and

th'ef time, delay rin :the circuit-together withithe `r90` .degree phase lagiimposedby the inductanceresistance correcting.A circuit, the amount .of feedbackif; allowable-.s without encountering .sustained oscillation? is smalL so that the overall correction `front,theifrequency to. phase modulationll'is-small.

' YIrr'n'ly arrangement, Yas illustratedin'li'ig. l, there arey nov filter fcircuits..` or. circuits'qof a similar' vnature `irca .feedback circuit:y K'Ihewdiscriminator circuit.- isqcoupled. :directly to Athe z converter asso'- ciated: Withthe modulated: Lheterodyningoscillations:V Inthearrangement ofEi-gs; .3f, .4;1` and1:5, separate paths areE :used so that there again.- ,the

lteiwcircuits-of 'the receiver arefnot inclu'dedfin theufeedrbackfpatha.

What is claimed is:

1.' Thef method; of .demodulating phase: modulatedf, wave energy whicngincludes 1: the Astepsf of der ncdulating: said-,'.w'aveY energy .torobtainmodulation potentials whichljare` modified; in :accordance with their frequency and therefore do not correctlyrepresentthemodulations on said wave energy, andl phase modulating the' said vwave energy in accordance with said demodulated p ctential's to causesaidmodulation potentials to correspond substantiallyto said wave modulations.

2. The method of demodulating phase modulated 'wave energy which `includes the steps of beating said wave energy with yoscillatory energy to obtain wave energy of lower frequency, demodulating said wave energy of lower frequency to obtain modulation potentials which are modied in accordance with their frequency and do not correctlyrepresent the modulations on said wave energy, and phase modulating the oscillatory energy used in said locating process with said 'potentials to cause said modulation potentials to correspond substantially 'to said wave modulations.

3. In means for demodulating phase modu-' lated wave energy, a frequency modulated wave detector having an input and an output, a phase modulator havingan input and having any output -coupled substantially directly to said frequency modulated wave detector input, a coupling between the output of said frequency modulated wave detector and s aid phase modulator, and means for impressing phase modulated energy on said phas'emodulator input.

4. In means for receiving phase modulated wave energy, afrequency m-odulatedv wave detector having an input'and an output, a heterodyne detector having an output coupled submeans f for :phase :modulating .the .oscillations impressed.' on said'converting;systemin accordance with, the:=output-of saidfrequency:A modulation detector.

6. In a phase modulation receiver, a frefquency modulated wave detector, a.v frequency converting system. having,v an input excited'I by said wave'energy,v an. oscillator.` for generating oscillati'ons,=.said converting system. having :an

output coupled 'tol-said. frequency modulated wave detector, connections for phase.. modulatingjY the .gener-ated#oscillations inaccordance with the outputrv of said Ifrequencyy modulation detector,

, and.. connections for multiplying: theA frequency ofathef phasef modulated generated oscillations andimpressingrthem on` said converting 1 system. 7.'.Inv a wave.l lengthwmodulation system, a

lsource of waves-lengthmodulated. wave zenergy,

a wave length modulated wave detector, connections for4 deriving-'potentials representative of llthexwave length modulations onsaidf'wave: en-

ergy; connections* forI additionally modulating the lengthy-of saidzwave energy inf accordance with said5 derived potentials; and additional con.-

nections: for impressingqsaid. .additionally moduv:lated .wave energy on. said idetector. 85SInlap.phase-.modulationsystem, a paircf frequencyw modulated# wave. detectors, connec- A.tions` for :impressing: phasemodulated waves on both of said detectors, and connections for additionally wave length modulating the waves impressed on one of said detectors in accordance with potentials derived fromv the output of the other `of said detectors.

`9. In a phase modulation receiver, a pair of I frequency modulated wave detectors, a pair of heterodyne' detectors including local oscillators for converting the phase modulated vwave energy to correspondingly modulated wave energy of lower frequency, connections between the heterdyne detectors and the frequency modulated wave detectors for impressing the said wave energy of lower frequency on said frequency modulation detectors, and connections between one of said frequency modulated wave detectors and one of said local oscillators for modulating the their frequency and, therefore, do not correctly' represent the modulations on said wave energy, and additionally modulating the phase of said wave energy in accordance with said derived potentials to cause said derived potentials to correspond. substantially fully with the modulations used to phase modulate said Wave energy.

l1. The method of deriving themodulation potentials from wave energy the phase of which has been modulated in accordance with said modulation potentials which includes the following steps, demodulating said phase modulated Wave energy to obtain therefrom modulation potentials which are modified in accordance with their frequency and, therefore, do not correctly represent the modulations on said wave energy,` additionally modulating the phase of said Wave energy in accordance with said derived potentials, and demodulating said additionally phase modulated wave energy to derive potentials which correspond substantially fully with the modulations used to phase modulate said first mentioned wave energy.

12. In apparatus of the class described,a converter circuit having an input whereon phase modulated wave energy is impressed, said converter circuit having an output, a frequency modulated wave detector having an input coupled to said converter and having an output from which the detected signal may be derived, a source of local oscillations, a phase modulator and a frequency multiplier in cascade coupling said source of oscillations to said converter, and a connection between the output of said detector and said phase modulator for modulating the oscillations therein in accordance with the output of the detector.

13. In apparatus of the class described, a first mixer stage having an input on which phase modulated energy to be demodulated is iinpressed, a first source of oscillations coupled to said mixer stage, said mixer stage having an output, an intermediate frequency amplifier having an input coupled to the output of said first mixer stage, said intermediate amplifier output having an output, a first frequency modulated Wave detector having an input coupled to the output of vsaid amplier, said detector having an output, a

second mixer stage having an input excited by said phase modulated wave energy, a second source of oscillations coupled to said second mixer stage, said second mixer stage having an output, a second intermediatefrequency amplifier having an input coupled to the output of said second mixer stage, said second amplifier having an output, a ysecond frequency modulated Wave detector having an input coupled to the output of said second amplifier, said second detector having an output from which the signal modulations may be derived, and a coupling between the output of said first frequency modulation detector and said second oscillator for modulating the phase of the oscillations generated by said second oscillator in accordance with the output of said first detector.

14. Iny apparatus of the class described, a rst Vconverter stage having an input excited by phase modulated wave energy to be demodulated, a source of oscillations coupled to said first converter, said first converter having an output, an

, amplifier having an input coupled to the output of said first converter, said amplifier having an output, a first frequency modulated wave detector having an output and having an input coupled to the output of said amplifier, a second converter having an input excited by said phase modulated wave energy, a second source of oscillations, a phase modulator and a-frequency multiplier connected in. the order recited for supplying beating oscillations to said second converter, a coupling between said phase modulator and the output of said rst detector for modulating the phase of the oscillations from said second source in accordance with the output of said first detector, and a second frequency modulated wave detector having an input coupled to said second converter and having an output from which the modulation may be derived,

MURRAY G. CROSBY. 

